AMV virus is a mixture of three viruses: a leukemogenic agent and two myeloblastosis associated helper viruses, MAV-1 and MAV-2. The ratio of the helpers to AMV is about 10:1. The genomic size of the two helpers was determined to be 7500 basis and that of AMV 7000. Complete DNA duplexes of genomic RNAs have been synthesized in vitro. The duplexes were estimated to be 5.2 and 4.0 x 10 to the 6th power daltons for helper and AMV respectively. Duplex AMV fragments synthesized in vitro were inserted into pBR322 and cloned in an approved EK2 host. 28 clones containing AMV inserts were isolated. These clones represent specific segments of the viral genome. cDNA probes prepared from AMV (MAV) were hybridized to a Southern blot restriction fragment for cell line Q5 DNA. This line consists of quail cells that have been transformed by MC29 but do not produce virus. The probe hybridized predominately to a single Bam HI fragment and a single Eco RI fragment which suggests that the integrated provirus within Q5 is located at a single site that is stable through cell division.